A type of gas-blast circuit breaker is disclosed in EP-A-0,380,907. When switching-off large currents, quenching gas is flowing into an arcing or blow-out space supporting the drive. In order to prevent an overpressure in the arcing or blow-out space during switching-on, which overpressure requires increased work from the drive, slide-like valves are provided which release passages, running in the radial direction, in the cylinder bounding the arcing space during switching-on as a consequence of overpressure produced in the arcing space, with respect to the pressure in the pump space, in order to ensure pressure equalization between the arcing space and the surrounding space. In order to control these valves, a control piston is displaceably supported in the pump piston which separates the pump space from the blow-out or arcing space. This control piston has further passages which connect the arcing space to the pump space and are closed by means of a valve element which opens automatically in the event of an overpressure in the arcing space with respect to the pump space, in order to allow quenching gas to flow into the pump space during switching-on.
This known circuit breaker has the disadvantage that a considerable pressure difference must be built up between the pump space and the arcing space in order to open the slide-like valves, which requires drive energy. Also, at the start of a switching-off stroke, the slide-like valves must initially be moved into a closed position again resulting in the pump space being enlarged by the displacement of the control piston. This has the consequence that the pressure built up in the pump space is less than necessary to avoid adversely affecting the switching-off capability of the circuit breaker. Furthermore, such slide-like valves result in a complicated construction of the gas-blast circuit breaker.